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re: decode CollisionSet_*.bin -- the per-object collision-mesh library

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All 18 blobs are byte-identical: the per-stage naming is nominal, and every
stage points at one shared 1675148-byte library stored eighteen times.  That
identical size was the reason to open the item, and it turned out to be the
answer to it.

Record layout: {u32 size, u32 name_len, char name[name_len], u32 nv, u32 nt,
f32[3] x nv, u32[3] x nt}, next record at off + 8 + size.  The indices are u32
here where MCOL uses u16 -- two different serialisers in one archive.

What makes this a decode rather than a plausible reading: the walk consumes the
file to the byte over 158 variable-length records, with the size word predicted
from the two counts 158/158.  A wrong field would desynchronise within a few
records and could not land exactly on the end.  All indices in range 158/158;
98.24% of edges shared by exactly two triangles; 147/158 fully manifold.

158 meshes, 90 836 triangles: per-part ship proxies (_bdy/_brg/_eng/_wep/_sld,
the XBG7 sub-part vocabulary) plus 46 stage asteroid meshes whose prefixes are
exactly the stages that have an _AsteroidVolume_wp MCOL.

Two things this file makes me walk back:

  * The "1 unit = 1 metre" reading from mapmesh_box_500km is downgraded to
    amber.  The 500000 arithmetic stands, but it implies that a craft the game's
    own tables call "small" is 133 m and that rob_f002 is 447 km -- 89% of the
    arena width.  The format check survives; the interpretation has no
    independent support.
  * The _cmesh <-> render-model name link is recorded as UNTESTED, not
    confirmed: only one .xbg build manifest survives on the disc, so matching
    stems against object names covers 4 of 158, which is no coverage at all.
This commit is contained in:
Sylpheed RE agent
2026-08-26 09:31:22 +00:00
parent 80ae860e19
commit d677bcfd80
5 changed files with 430 additions and 5 deletions
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13
docs/re/structures/mcol-collision.md
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@@ -540,9 +540,16 @@ triangles spanning exactly ±250 000** — a cube. Its name says that cube is
> **one world unit is one metre.**
This is a name-based inference, but the arithmetic is exact and it runs the
other way as a check on the format work: a wrong stride or index width could not
have produced a box whose measured size matches its own filename.
The arithmetic is exact and it runs the other way as a check on the format work:
a wrong stride or index width could not have produced a box whose measured size
matches its own filename.
> 🟡 **Downgraded 2026-08-26, same day.** The *format* check stands. The
> *interpretation* does not: decoding `CollisionSet_*.bin` gave the first chance
> to see what a metre implies, and it makes a craft the game's own tables call
> "small" 133 m long and the largest object 447 km — 89 % of the arena. Either
> the ships really are kilometre-scale or `500km` names something other than the
> span. See [`collisionset.md`](collisionset.md).
Reproduce with `tools/re-capture/miscbin_names.py <extract root> [--pairs]`
(recorded in [`../data/miscbin-names.txt`](../data/miscbin-names.txt)).